Chemical vapor deposition (CVD) systems can be used to deposit thin films on substrates by decomposing vapor precursors within low-pressure reactors. The vaporization of the precursors takes place prior to entry of the precursors into the reactors.
Purified forms of metals, metal compounds, and other materials can be deposited in uniformly thin layers onto substrates by decomposing vaporized precursors of the materials. The depositions take place inside reactors with evacuatable environments and temperature controls. Many of the precursors take liquid form at ambient temperatures and are vaporized at higher temperatures just prior to entry into the reactors.
High deposition rates for such chemical vapor deposition (CVD) processes require correspondingly high delivery rates of vaporized precursors into the reactors. Vaporization of liquid precursors can be carried out by mixing the liquid precursor with a carrier gas or by atomizing the liquid precursor in a suspended gas. Liquid flow rates into vaporizers are limited by conversion capabilities of the vaporizers to vaporize the liquid precursors. Incomplete vaporization can result in the passage of large droplets of the liquid precursor into the reactors. The entry of liquid precursor into reactors, which is referred to as xe2x80x9cfloodingxe2x80x9d, contaminates the reactors and diminishes pumping performance. Flooding increases deposition processing time by requiring more time to evacuate the reactors.
Our invention provides opportunities for vaporizing liquid precursors at high rates and for delivering the vaporized precursors to low-pressure reactors for processing, while preventing the delivery of any remaining liquid precursor to the reactors. Droplets of liquid precursor remaining after a first stage of vaporization are trapped and subject to a second stage of vaporization. More efficient vaporization enables the higher rates of vaporization to be achieved. Throughput processing rates can also be improved by avoiding passage of liquid precursor droplets into the reactors.
One example of a precursor vaporizer for a chemical vapor deposition system has an inlet arrangement for admitting a liquid precursor and a carrier gas into the vaporizer. A first vaporizing stage vaporizes a portion of the liquid precursor into the carrier gas. A second vaporizing stage located gravitationally below the first vaporizing stage vaporizes another portion of the liquid precursor into the carrier gas. A vaporization chamber interconnects the first and second vaporizing stages. An outlet conveys the vaporized precursor from both vaporizing stages to a reactor of the chemical vapor deposition system. The outlet is connected to the vaporizing chamber out of liquid communication with the first vaporizing stage and extending gravitationally above the second vaporizing stage to prevent the remaining liquid precursor from reaching the reactor.
The inlet arrangement preferably includes separate conduits that support flows of carrier gas through both vaporizing stages towards the vaporizing chamber. The flows of carrier gas supported by the inlet arrangement can include (a) a first flow of the carrier gas through the first vaporizing stage in a direction corresponding to a gravitationally directed seepage of the liquid precursor through the first vaporizing stage and (b) a second flow of the carrier gas through the second vaporizing stage in a direction opposed to a gravitationally directed seepage of the liquid precursor through the second vaporizing stage.
A separator within the vaporization chamber can be used to allow the liquid precursor to reach the second vaporizing stage and to allow the vaporized precursor to pass through the outlet. In addition, the separator can prevent the liquid precursor from passing through the outlet, preferably by diverting the liquid precursor from the outlet. For example, the separator can be formed as a roof over the outlet with pervious under-eaves structure for admitting the vaporized precursor under the roof.
The inlet arrangement also preferably includes a mixing valve that mixes the liquid precursor with the carrier gas in advance of the first vaporizing stage. The mixing valve regulates flow rates of the liquid precursor into the vaporizer. A signal from a flow meter to the mixing valve can be used to adjust the flow rates of the liquid precursor into the vaporizer.
The two vaporizing stages and the intermediate vaporizing chamber are preferably supported within a thermally conductive body that supports transfers of heat to the vaporization process. However, the mixing valve is preferably supported by a thermal isolator for insulating the mixing valve from the thermally conductive body. One or more heating elements positioned within the thermally conductive body heat the first and second vaporizing stages without substantially heating the mixing valve.
The second vaporizing stage preferably includes a trap for capturing the liquid precursor below a level of the outlet and a porous medium within the trap to increase surface area. A carrier gas passageway provides for conducting carrier gas through the porous medium to vaporize the liquid precursor captured in the trap. Preferably, the carrier gas passageway is arranged to convey the precursor vaporized by the second vaporizing stage in a direction opposed to gravity en route to the outlet in the vaporizing chamber.
During operation, the mixer preferably combines a liquid precursor with a carrier gas at a first temperature low enough to avoid significant decomposition of the liquid precursor. The first and second vaporizing stages promote vaporization of the liquid release agent at a second temperature high enough to avoid significant condensation of the vaporized precursor. The mixing is preferably carried out at ambient temperatures to prevent the mixing valve from becoming clogged with prematurely decomposed solids. The vaporizing stages, however, are preferably heated well above ambient temperatures to prevent condensation of the vaporized precursor.
A precursor for a low-pressure processing system can be vaporized in accordance with our invention by a series of steps for increasing vaporization efficiency and overall processing rates. A liquid precursor and a carrier gas are admitted into a vaporizer. A portion of the liquid precursor is vaporized into the carrier gas at a first vaporizing stage. A remaining liquid portion of the precursor from the first vaporizing stage is separated from the vaporized portion of the precursor. The remaining liquid portion of the precursor is passed to a second vaporizing stage. At least a portion and preferably all of the remaining liquid portion of the precursor are vaporized at the second vaporizing stage. The vaporized precursor from both vaporizing stages is passed through an outlet located gravitationally below the first vaporizing stage and gravitationally above the second vaporizing stage.
Preferably, the admission of the liquid precursor and the carrier gas includes mixing the liquid precursor with the carrier gas at a temperature low enough to avoid significant decomposition of the liquid precursor. Flow rates of the liquid precursor into the vaporizer can be regulated by a mixing valve that accepts a feedback signal from a flow meter. The mixing valve is preferably thermally isolated from the first and second vaporizing stages to conduct the mixing operation at ambient temperature.
Both vaporizing stages conduct flows of the carrier gas in opposite directions. The carrier gas is conducted through the first vaporizing stage in a direction corresponding to a gravitationally directed seepage of the liquid precursor through the first vaporizing stage and the carrier gas is conducted through the second vaporizing stage in a direction opposed to a gravitationally directed seepage of the liquid precursor through the second vaporizing stage.
The separation of the two states of the precursor between vaporizing stages preferably includes allowing the vaporized precursor from the first vaporizing stage to pass through the outlet and preventing the remaining liquid precursor from the first vaporizing stage from passing through the outlet. The liquid precursor remaining from the first vaporizing stage is preferably diverted from the outlet to the second vaporizing stage. The separation preferably takes place within a vaporization chamber interconnecting the two vaporizing stages. The vaporized precursor from both vaporizing stages preferably passes through the same outlet within the vaporizing chamber.
The remaining liquid portion of the precursor from the first vaporizing stage is captured in a trap at the second vaporizing stage below a level of the outlet. The carrier gas is preferably conducted through a porous medium within the trap to vaporize the liquid precursor captured in the trap. The carrier gas is flowed through the trap in a direction opposed to gravity en route to the outlet.